First off, that is a very nice aquascape. Well done.
Now to respond to your questions. What you describe in post 2 is likely spawning behaviour. Changes in water parameters such as what occurred from the store tank to your aquarium can often trigger this, because it replicates what occurs in the fish's natural habitat. Spawning with forest fish almost always occurs at the start of the rainy season when the streams are inundated with cooler water but also water with a very different pH. Difficult species can often be spawned by substantially altering the water parameters.
As for fry surviving, possible but unlikely. In very thickly-planted tanks [let those plants really grow in
] eggs will be scattered in vegetation [Java Moss
is ideal for this] and often survive predation if the fish are few and well fed. Fry can find microscopic plankton in heavily-planted tanks and will suddenly appear. Usually just a few, unless you specifically work to raise them with infusoria, removing the other fish, etc. Some fish are more proficient at this than others. I have dozens of Emperor Tetra
fry that have survived over the past several months from many spawnings. They just suddenly show up, usually 2, 3, or 4 at about 1/4 inch when I first notice them. I have had one fry appear from many species of characin over the years. Another benefit of well-planted tanks.
The fish sound fine. The one dead may well have been due to injury, it is very easy to injure fish with a net, I have done it. If the others do well, after a couple days i would see if the store will replace the one. Some stores ask for the "corpse" and a water sample, some don't. Worth a try; after all, they want your continued business.